Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Jan 01 2024 Valid 12Z Thu Jan 04 2024 - 12Z Mon Jan 08 2024 ...Overview... The medium range period features a rather progressive flow pattern as a parade of storm systems traverses the country from West to East. One will be exiting the East Coast as another moves through the Southwest on Thursday, with potential for another round of heavy rain across the Lower Mississippi Valley/Southeast by Friday. As this system lifts up the East Coast next weekend, it has potential to bring heavy precipitation (rain to the south, snow to the north) to much of the East. Out West, the next system looks to move in next weekend as well with another round of valley rain and mountain snows progressing from California into the Four Corners region. ...Guidance/Predictability Assessment... The latest guidance continues to show good agreement on the overall synoptic pattern during the Thursday-Monday medium range period. There are some more sensitive differences in the details and timing of individual systems. The biggest uncertainty is with developing low pressure across the Gulf Coast which models are increasingly suggesting may strengthen as it lifts along the East Coast next weekend. There remains considerable spread in the exact track and timing of this system, which has huge implications for precipitation type across some of the major cities of the Mid-Atlantic/Northeast. Elsewhere, some differences with northern stream shortwaves through the Great Lakes and also with the second trough entering the West/Southwest next weekend and potential low pressure development across the south-central Plains next Monday. The WPC forecast for tonight used a blend of the deterministic guidance early in the period, increasing contributions from the ensemble means beginning on day 5 to help smooth out some of the differences with the East Coast system. This also helped to maintain fairly good continuity with the previous WPC forecast for this storm as well. Leaned more on the ECMWF and ensemble means late period across the West. ...Weather/Hazards Highlights... Precipitation across the Four Corners/southern-central Plains to start on Thursday should become more widespread in nature as the first of a series of upper lows taps Gulf of Mexico moisture to fuel another round of moderate to locally heavy rainfall across the Lower Mississippi Valley/central Gulf Coast states on Friday. There is enough instability and moisture to support a marginal risk across this region on the Day 5 Excessive Rainfall Outlook, especially considering the potential for modest rainfall across parts of this region during the short range as well. Heavy precipitation will spread across the Southeast/Mid-Atlantic/Northeast next weekend, with increasing potential for snow to the north and west of the low. Exact amounts and precipitation type for some of the major East Coast cities remains highly uncertain at this point, but does need to be watched as there is potential for some modest snows across at least interior portions of this region, as highlighted by the WPC winter weather outlook, which could affect the DC-NYC corridor depending on the track of the low. The next system moves into the western states next weekend spreading moderate to locally heavy precipitation across the West Coast/California and into the interior West next Monday. Some signal for decent mountain snows across parts of the Cascades, Sierras, and interior mountain ranges/southern Rockies. Surface cyclogenesis across the south-central Plains early next week should allow precipitation to move into the central U.S., with potential for snow to the north of the low track. Above normal temperatures should be mostly confined to the far northern tier with anomalous generally +5-15F. Some moderately above normal temperatures may return to the Southern Plains/Lower Mississippi Valley early next week. Near to below normal temperatures should spread across the East into the weekend, with temperatures also trending much cooler across the West as upper troughing moves into the region late in the period. Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw